Dimmers, typically in the form of dimmer switches, are devices used to vary the intensity of light output by one or more lamps. Conventional dimmer switches typically vary the intensity of light output by increasing or decreasing the root-mean-square (“RMS”) voltage, and hence the mean power, provided to the lamp. To adjust the RMS voltage, conventional dimmers cut off either the leading edge or the trailing edge of an alternating current (“AC”) input sinusoid. The intensity of the light output is determined by the proportion of the AC input sinusoid that is applied across the lamp. The portion that is cut off is commonly referred to as the dead time.
One common dimmer design employs a triac for cutting off a portion of one of the edges of the AC input sinusoid. Specifically, when a triac is electrically coupled to an AC source, the triac blocks the flow of current until the triac is triggered. A typical triac dimmer uses a gating circuit having a variable resistor, capacitor, and a diac. The triac is triggered when the voltage level of the capacitor charges to the nominal voltage of the diac, typically about 30 volts. The time for the capacitor to charge to the trigger point is set by the capacitance of the capacitor and the resistance of the variable resistor, such that a low resistance decreases the dead time (e.g., bright mode) and a high resistance increases the dead time (e.g., dim mode). The resistance of the resistor is controlled by a dimmer switch (e.g., slider switch or rotary switch). That is, the setting of the dimmer switch controls the amount of resistance of the variable resistor in the gating circuit. Once triggered, the triac continues to allow current to flow until the current through the triac reaches a level of zero. In short, the triac cuts off the leading edge of the input AC sinusoid. As the dimmer switch moves such that the operator of the switch would expect a lamp attached to the dimmer switch to grow dimmer, the triac is triggered later in the cycle, and the portion of the sinusoid that is cut off increases. As the portion of the sinusoid that is cut off increases, the RMS voltage of the AC input delivered to the lamp decreases.
However, the resistance of the resistor must be kept above a certain level to avoid damage to the trigger circuit components. Furthermore, the resistor is charged from the power source voltage so the power source voltage must be a minimum of the trigger point voltage (e.g., 30 volts) before the trigger circuit can operate. This causes the dimmer to remove an initial portion of the AC input sinusoid supplied to the load even when the dimmer switch is set for full power output. Therefore, conventional triac dimmer switches are not capable of providing full output voltage to lamps even when the dimmer switch is adjusted to the full output position. That is, even when a conventional triac dimmer switch is adjusted for full output voltage, the dimmer switch still cuts off a portion of the AC input sinusoid, resulting in less than full output RMS voltage.
This results in inefficient lighting and can sometimes require additional lamps or employing lamps with a higher power rating to correct. This can be very costly, especially for high efficiency LED lighting. It is also undesirable to customers to increase the number of lamps or the power rating of each lamp in order to restore proper lighting.
This inability to provide full voltage output also presents a problem for lighting designers. Lighting designers typically design lighting layouts based on manufacturer specifications that indicate total lumen output for a lamp or light fixture. However, if the lighting designer plans to install a triac dimmer circuit for the lamp or fixture, the designer is going to get a portion of the rated output only, resulting in inadequate illuminance on the illuminated object.